The time-honored skid avoidance technique of "pumping" a brake pedal to control a skid situation has largely been displaced by anti-lock braking systems. Many known anti-lock devices operate by cyclically increasing and decreasing a braking force exerted on the wheels so that a slipping wheel having a tendency to lock is permitted to re-accelerate back to a spoed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into a brake cylinder to first lower and then raise the brake pressure in the brake system.
Typically anti-lock or anti-skid braking systems utilize a so-called pump-back scheme or a replenishing scheme during a reapply or build operational sequence to maintain a desired level of hydraulic fluid in a brake system. In a pump-back scheme, the same hydraulic fluid is re-supplied from a local accumulator to the brake pad actuators while in a replenish scheme hydraulic fluid comes from a separate source such as either a hydraulic accumulator or a separate pump and motor.
Most of such anti-lock braking systems are further capable of operating in a traction control function. A traction control function is established by detecting conditions where the rotational speed of a first powered wheel substantially exceeds that of a second powered wheel. To provide a power balance in the operation of a vehicle, a braking force is applied to the powered wheel rotating at a higher speed to effectively transferring driving torque back to that wheel with better traction. Many anti-lock systems having such a traction control feature employ a motor and hydraulic pump or pumps which operate independent of the service braking system. Such a braiding system is disclosed in U.S. Pat. No. 5,709,438 wherein a traction control motor and hydraulic pump are called into action to provide a back-up power assisted braking feature in the event of a malfunction of the primary braking system. Such malfunctions may occur simply because the engine is not operating to drive the primary power braking source, ruptured or failed brake lines, broken power steering pump drive belts, or for a variety of other reasons. This prior patent provides back-up anti-skid braking in but a single mode. This prior patent represents the point of departure for the present invention.
It is desirable to provide a braking control system of the type disclosed in the above mentioned U.S. Patent having a back-up feature which modulates pressure to the service brakes in the event of loss of the primary brake power source. It is also desirable to provide a "wake-up" mode of operation which allows secondary or back-up power assisted braking with the ignition off.